In a number of instances in which the wheel rotates on an axle, it is desirable to accurately determine the plane of rotation of the wheel, i.e. that plane which is defined by the circular locus of points traced by a particular point on the wheel as the wheel rotates. The orientation of this plane is uniquely defined using the axis of rotation of the wheel, i.e. an imaginary line perpendicular to the plane and passing through every diameter of the circular locus. Determination of the wheel plane is of use in connection with aligning the wheels of a multi-wheeled vehicle, e.g. motor vehicles such as automobiles, trucks and the like. Optimum performance of such vehicles is obtained when the wheels are positioned in a predetermined alignment, typically specified by the manufacturer in terms of toe, camber, caster and other angles. The toe and chamber angles specified by the manufacturers refer to preferred alignments of the plane of rotation of the wheel. Deviations greater than the recommended tolerance from the preferred alignment can result in poor handling and damage or excessive wear to the tires or other parts of the vehicle. It is evident, therefore, that is useful to be able to determine the orientation in space of each of the vehicle wheels, e.g., with respect to a reference plane or point. In this manner, it is possible to determine whether a vehicle is in accord with recommended alignment parameters and, during processes of adjusting such parameters, whether correct adjustment has been achieved.
To be useful in measuring and adjusting wheel alignment, methods and apparatus for measuring wheel planes must have a high degree of accuracy, and at least sufficient accuracy to assure that the maximum measurement error is no more than the recommended tolerance. At the same time, practical considerations dictate that wheel plane determination methods and apparatus do not require excessive space or time to operate. In the past, some amount of accuracy was sacrificed in order to minimize time or space or achieve other goals such as portability. U.S. Pat. No. 3,460,264, issued Aug. 12, 1969 to Clutchy, discloses a portable wheel alignment checking device having portions which engage areas of two opposed wheels and remain adjacent to those areas as the vehicle is moved to accomplish one rotation of the wheels. This device, however, gauges relative spacing of areas on opposed wheels and does not directly provide information on the wheel plane of a single wheel relative to a reference point or plane. U.S. Pat. No. 1,589,048, issued June 15, 1926 to Bussard, and U.S. Pat. No. 3,116,562, issued Jan. 7, 1964 to Hunter, both relate to obtaining measurements of distances between two wheels at a number of points along the wheel or tire. Neither of these references relates to measuring the plane of rotation of a single wheel relative to a reference point or plane. Other apparatus measures three points on a wheel simultaneously to define a plane. However, such apparatus is affected by both local aberrations such as small bulges on the tire or raised lettering and by systematic aberrations such as run-out, i.e., non-perpendicularity of the wheel with respect to the axle on which the wheel is mounted. This apparatus also provides too few data points per application to be practical for forming any statistical measure of reliability of the data. Another known apparatus determines the position of a number of points in a small region of the tire, taking the average of these points as an indication of the true position in space of a point on the tire. However, mere averaging does not provide a statistical measure of the reliability of the data and the apparatus, in practical terms, can provide only a finite number of data points.
Accordingly, it would be advantageous to provide a method and apparatus which can provide information on the orientation in space of the plane of rotation of a single wheel relative to a reference point or plane while requiring only a small physical space to operate and requiring little operator time for set up, orientation of the apparatus with respect to the vehicle, or other operating procedures. It also would be advantageous to provide a method and apparatus which can generate a statistical measure of the reliability of the data it has produced and which is capable of generating any number of data points needed to achieve a predetermined reliability of data.